MOTA v6.10 USER MANUAL

TABLE OF CONTENTS

INTRODUCTION **

REGISTRATION AND HELP **

1 ... COMPUTING PLATFORM AND INSTALLATION **

1.1 HARDWARE **

1.2 OPERATING SYSTEMS **

1.3 INSTALLATION **
 
 

2 ... RUNNING MOTA **

3 ... THE MAIN MOTA MENU **

3.1 FILE **

3.2 RUN THE SIMULATOR **

3.3 TOOLS **

3.4 EXPANSION CHAMBER CONSTRUCTION **

3.5 POWAPLOT **

3.6 HELP **

3.7 EXIT FROM MOTA **

4 ... FILE MENU: NEW ENGINE DATA FILE/EDIT EXISTING ENGINE DATA FILE **
4.1 ENGINE CONFIGURATION **
4.1.1 Induction Type *

4.1.2 Exhaust System Type *

4.1.3 Number of Transfer Port Types *

4.1.4 Induction Box *

4.2 BASIC ENGINE DIMENSIONS **
4.2.1 Bore, Stroke, Connecting Rod Length *

4.2.2 Cylinder Clearance Volume *

4.2.3 Crankcase Clearance Volume *

4.2.4 Gudgeon Pin Offset *

4.3 IGNITION AND COMBUSTION DETAILS **
4.3.1 Combustion Efficiency *

4.3.2 Burn Period *

4.3.3 Defining The Ignition Timing Advance Characteristic *

4.3.4 Ignition Advances*

4.4 AMBIENT CONDITIONS **
4.4.1 Ambient Pressure and Temperature *

4.4.2 Water Cooled Exhaust *

4.5 FUEL AND SCAVENGE DETAILS **
4.5.1 Fuel Calorific Value (i.e. Fuel Heating Value) *

4.5.2 Throttle Setting (i.e. Throttle Area Ratio) *

4.5.3 Number of Points Defining the Air/Fuel Ratio Characteristic *

4.5.4 Air/Fuel Ratio(s) *

4.5.5 Scavenge Parameters *

4.6 SIMULATION RUN PARAMETERS **
4.6.1 Output Units *

4.6.2 Comment for Performance File Header *

4.6.3 Pipe Step Factor *

4.6.4 Number of Engine Speeds *

4.6.5 Engine Speed or Initial (lowest) Engine Speed *

4.6.6 Speed Increment *

4.6.7 Speed for Graphics Output *

4.6.8 Maximum Number of Revolutions at Each Speed *

4.7 PISTON CONTROLLED PORT SCREENS **
4.7.1 Data Common to Both Rectangular and Profiled Ports *

4.7.2 Rectangular Ports *

4.7.3 Profiled Ports *

4.8 INLET ROTARY VALVE DETAILS **
4.8.1 Number of Rotary Valves *

4.8.2 Opening Angle *

4.8.3 Closing Angle *

4.8.4 Radius to Bottom of Port *

4.8.5 Port Height *

4.8.6 Angular Port Width *

4.8.7 Corner Radii *

4.9 INLET REED VALVE DETAILS **
4.9.1 Number of Reed Valves *

4.9.2 Valve Block *

4.9.3 Uniform/Stepped Petal Thickness (Single and Compound Petals) *

4.9.4 Petals *

4.10 DUCT SCREENS **
4.10.1 Data Common to all Ducts *

4.10.2 Inlet and Induction Ducts *

4.10.3 Exhaust Duct *

5 ... FILE MENU: VIEW AN ENGINE PERFORMANCE FILE **
5.1 DEFINITIONS OF ENGINE PERFORMANCE MEASURES **
5.1.1 Compression Ratios *
5.1.1.1 Cylinder Compression Ratio *

5.1.1.2 Crankcase Compression Ratio *

5.1.2 Power Output and Torque *

5.1.3 Mean Effective Pressures *

5.1.3.1 Brake Mean Effective Pressure (BMEP) *

5.1.3.2 Pumping Mean Effective Pressure (PMEP) *

5.1.3.3 Friction Mean Effective Pressure (FMEP) *

5.1.3.4 Indicated Mean Effective Pressure (IMEP) *

5.1.4 Brake Specific Fuel Consumption *

5.1.5 Duct Flow Ratios *

5.1.6 Scavenge Ratios *

5.1.6.1 Mass Based Scavenge Ratio *

5.1.6.2 Volume Based Scavenge Ratio *

5.1.7 Efficiencies Related to the Scavenge Process *
5.1.7.1 Scavenge Efficiency *

5.1.7.2 Trapping Efficiency *

5.1.7.3 Charging Efficiency *

5.1.8 Percentage Energy Loss in the Exhaust System *
5.2 SIMULATION PARAMETERS **

5.3 ENGINE GEOMETRY **

5.4 FUEL PARAMETERS **

5.5 COMBUSTION PARAMETERS **

5.6 AMBIENT CONDITIONS **

5.7 PISTON PORT DIMENSIONS **

5.8 PISTON PORT PROFILE DETAILS **

5.9 PISTON PORT TIMING DETAILS **

5.10 ROTARY VALVE PORT DATA **

5.11 REED VALVE PORT DATA **

5.11.1 Block Dimensions *

5.11.2 Petal Dimensions *

5.11.3 Port Dimensions *

5.12 DUCT DIMENSIONS **

5.13 ENGINE PERFORMANCE INDICATORS **

6 ... FILE MENU: DISPLAY ENGINE GRAPHICAL INFORMATION **
6.1 POWER/TORQUE **

6.2 WAVES **

6.3 CYLINDER-EXHAUST PRESSURE DIFFERENCE **

6.4 INDUCTION REED VALVE PETAL LIFT **

6.5 SCAVENGE RATIO **

6.6 DELIVERY RATIOS **

6.7 BOX CHARACTERISTICS **

6.8 DUCT CHARACTERISTICS **

7 ... RUNNING THE ENGINE SIMULATOR **

8 ... ENGINE DATA FILE CONSTRUCTION TOOLS **

9 ... EXPANSION CHAMBER CONSTRUCTION UTILITIES **

9.1 CONSTRUCTING THE DEVELOPMENT OF A CONE **

9.2 PRINTING THE DEVELOPMENT PATTERN OF A CONE **

10 ... POWAPLOT **
10.1 CONSTRUCT A POWER FILE FROM A GRAPHICS OUTPUT FILE **

10.2 CONSTRUCT A POWER FILE MANUALLY **

10.3 DISPLAY A POWER FILE **

11 ... DEMONSTRATION ENGINE DATA AND SIMULATION OUTPUT FILES **

CONVERSION FACTORS **

CUSTOMER SUPPORT **

DISCLAIMER **

TRADEMARKS **

DEDICATION **

ACKNOWLEDGEMENTS **

INTRODUCTION

This manual refers to the Windows version of MOTA, characterised by version numbers 6.00 and beyond.

MOTA is the result of on-going collaboration between University research engineers and mathematicians, and Ian Williams Tuning. As further research and validation is completed, updated versions of MOTA will be made available.

The term MOTA is used to cover the complete suite of programs which build, modify and run Engine Data Files, and process Graphics Output Files and Engine Performance Files.

MOTA has been developed to simulate the performance of single cylinder two-stroke engines. MOTA allows the simulation of engines with reed valve, rotary valve or conventional piston port timed induction systems and expansion chamber exhaust systems (with either an integrated go-kart type muffler or a separate muffler) or box muffler exhaust systems.

Because MOTA solves the equations describing the conservation of fluid and thermodynamic properties throughout an engine, it requires the specification of the full engine geometry. This is accomplished through a menu driven environment which prompts you for the required dimensions. This manual describes each of these inputs in detail.

The simulator in MOTA uses data supplied from an Engine Data File. Such a file is constructed by completing a set of screens which prompt you for information about your engine. The simulator outputs information to a text based Engine Performance File which summarises the engineâ€s geometry and performance, and to a Graphics Output File which is used to depict various aspects of the engineâ€s performance graphically. Facilities within MOTA allow you to view this information.

The strength in MOTA lies not so much in its ability to predict accurately the performance of an engine, but in its ability to compare the performances of different engine configurations. For example, if you have the MOTA generated power curve for an engine, and you want to see if modifying one of the exhaust pipe dimensions will increase or decrease power, then you can change that particular dimension and re-run the MOTA simulator. Comparison of the new power curve with the old will allow you to determine whether or not the change will be beneficial.

MOTA will simulate one of the cylinders of a multi-cylinder two-stroke engine provided that the cylinders are identical in layout and dimensions and each cylinder has a separate exhaust system and a separate induction system.

REGISTRATION AND HELP

Please remember to complete the registration card and to post or fax it to us. This will allow us to keep you informed of upgrades and of any MOTA user newsletters we might produce.

1 ... COMPUTING PLATFORM AND INSTALLATION

1.1 HARDWARE

MOTA will run only on IBM compatible computers. It is recommended that a minimum configuration is a Pentium machine with a clock speed of 233 MHz and 32 Mb of RAM. MOTA requires a VESA compatible VGA video adapter in your computer.

This version of MOTA was developed using an 800 x 600 screen resolution colour display and small fonts. It will satisfactorily run on higher resolutions, but monitors below 15 inches in size may have their displays overfilled by some of the MOTA screens if a resolution of 640x480 is used. For this reason, the latter resolution is not recommended.

A CD reader is necessary to install the MOTA software.

1.2 OPERATING SYSTEMS

This version of MOTA will run under WINDOWS 95, WINDOWS 98, WINDOWS 2000, WINDOWS NT, WINDOWS ME and WINDOWS XP.

1.3 INSTALLATION

MOTA cannot be run from the CD, it must be installed on a hard drive.
Full instructions for installing the MOTA software are provided in the Readme.htm file contained on the distribution CD. This file contains also details of the various procedures for Running MOTA and for Accessing the MOTA Manual as well as sections concerning Printer Installation and the Demonstration Slide Show.

2 ... RUNNING MOTA

In order to run MOTA you need to plug the Hardlock (dongle) into one of the USB ports or into the parallel printer port of your computer according to the type of dongle supplied. The dongle must remain plugged into your computer whilst you are running MOTA and it may remain plugged into your computer at all times with no undesirable effect.

For the dongle which plugs into the parallel printer port, your printer cable can be plugged into the back of the dongle and this should not interfere with your printing facility. On some computers, when the dongle is removed, the cable restored and then, for example, if a Microsoft Word document is printed, the printer port may lock. Whilst a computer shutdown and reboot will solve this problem it may be prudent to either leave the dongle permanently plugged into your computer or to remove and attach it only whilst your computer is shut down.

To start MOTA, double click the MOTA icon installed on your desktop during the program installation.

If you receive any messages relating to the dongle being missing, check that it is correctly plugged into the appropriate port. If, with the dongle properly connected, you still receive a message to the effect that the dongle is missing, please contact your distributor.

The Engine Simulator is the most important component of MOTA.

The principal function of MOTA is the use of an Engine Data File by the MOTA Engine Simulator. The engine simulation process produces two files by which the user can examine the engine performance. The Engine Performance File is a text file whilst the Graphics Output File contains numerical values for use in the graphical display of the engineâ€s performance. The entire suite of programs which constitute MOTA enable:

3 ... THE MAIN MOTA MENU

When MOTA is started the Main MOTA Menu screen is displayed. The Menu Bar situated at the top of this screen contains six titles:
File, Run the Simulator, Tools, Powaplot, Helpand Exit from MOTA. Each is described briefly in the current section and comprehensively in later sections of this manual.

3.1 FILE

Clicking on File provides a menu comprising six selections:

  1. New Engine Data File. This allows you to specify the characteristics and dimensions of the engine you wish to simulate. Various screens can be selected from a main engine data file construction screen within this option, allowing you to enter your engineâ€s characteristics in a logical manner. When you select the Save the File option from this main screen, you must then enter a name for your engine file. The file is then saved to the folder of your choice, with a ".dat" extension. All files with this extension are recognised by MOTA as engine data files. Such files created by the current version contain internal identifiers which designates them as MOTA v6 data files. This allows MOTA to distinguish between valid data files and other files with an identical extension which are not MOTA v6 data files. Should you later try to load such an invalid file for editing (see 2 below) or for use by the MOTA simulator, you will receive a message to the effect that the file is unsuitable. For comprehensive details refer to section 4 of this manual.
  2. Edit Existing Data File. This allows you to select an existing file of the type constructed under New Engine Data File, edit it and then save it under its original name or under a new name. For comprehensive details refer to section 4 of this manual.
  3. View an Engine Performance File. Selecting this option after a simulation run allows you to look at the ".per" file written during an engine simulation. We recommend strongly that you do this. The Engine Performance File contains all of your input data and other values such as port areas as well as various measures of your engineâ€s performance. Also, because it is very easy to make typographical mistakes when entering data, the Engine Performance File provides a useful check of the correctness of the Engine Data File values. For comprehensive details, refer to section 5 of this manual.
  4. Display Engine Graphical Information. When an engine simulation is completed a file is created having the same name as the engine data file, but with the extension ".gph". This option allows you to examine your engineâ€s performance via both static and animated graphics. For comprehensive details, refer to section 6 of this manual.
  5. Engine Data File Conversion (v5 to v6). This utility is provided to assist those MOTA users who have upgraded from version 5 to version 6.
  6. Exit from MOTA. Shuts down the MOTA environment. This is an alternative to the exit facility provided as a separate option of the Main MOTA Menu.
Selecting any of the choices 2-4 will result in a dialogue box inviting you to select the drive, folder and file for the appropriate action. Only the relevant files are displayed in the file dialogue window: Files with the extension ".dat" for engine data files, those with ".gph" for Graphics Output Files, and ".per" for Engine Performance Files. You may select any file by double clicking it or by clicking it once and then clicking the "open" button in the dialogue box.

3.2 RUN THE SIMULATOR

This selection runs the engine simulator which is the heart of the MOTA package. The simulator accesses a data file with a ".dat" extension, performs an engine simulation run and writes two files with the same name as the engine data file but with the extensions: ".per" and ".gph". The first of these is the Engine Performance File which provides text information about the engineâ€s performance. The second file is the Graphics Output File. This provides information for a graphics post-processor which can be used to display both stationary and animated views of the engineâ€s performance. These two files are always written to the same drive and folder as that containing the engine data file used for the simulation. Comprehensive details of an engine simulation run are given in section 7 of this manual.

3.3 TOOLS

Clicking on Tools provides a menu of utilities for calculating and converting engine related parameters. An expansion chamber construction facility is also provided. These utilities are described fully in section 8 of this manual.

3.4 EXPANSION CHAMBER CONSTRUCTION

This utility is provided to assist the MOTA user who wishes to design and construct an expansion chamber. It can provide the dimensions required to draw the development of a cone to be used as a section of a straight expansion chamber. It can also be used to print the development pattern of a cone and in this case the cone may be angled rather than square with the cone axis. In addition, given the dimensions of a straight cone section, patterns of the pieces used to construct an equivalent angled section can be printed.

Further details of this utility are provided in section 9 of this manual.

3.5 POWAPLOT

This utility is provided to enable the easy comparison of the performance of an engine as measured by a dynamometer with that predicted by the MOTA engine simulator. It involves the construction and use of files having a ".pow" extension which contain a set of values of engine speed and power.

The selection of Powaplot provides a menu of three selections:

  1. Construct a Power File from a Graphics Output File. This selection allows you to extract the power curve information from a nominated Graphics Output File produced by a MOTA engine simulation and write it to a separate Power File with the extension ".pow".
  2. Construct a Power File Manually. You enter values of engine speed and the corresponding power or torque through the keyboard and then save this information to a file with a ".pow" extension. You are asked to nominate the input units and also whether the speeds will be in equal increments in which case only the initial speed and the increment are entered.
  3. Display a Power File. Under this selection you are asked to select a Power File. The power/torque curves representing the values in this file are then displayed. Many display options are provided such as the change of display units, the change of scales and the selection of other Power Files and/or Graphics Output Files to provide overlay curves to the display. The overlay facility allows the easy comparison of engine performance.
Comprehensive details of the Powaplot utility are provided in section 9 of this manual.

3.6 HELP

Clicking on Help provides a menu of two selections.

  1. User Manual . This selection provides an on line display of this manual using your computerâ€s web browser. The entire content or any selection of pages of the manual may be output to your printer.
  2. About. This screen provides fax and email contact details for any enquiries you might have concerning MOTA together with an associated website address. The website provides details of Ian Williams Tuning which has played a major role in the development of the MOTA software. As necessary, technical explanations, upgrade information and any other matters of concern to MOTA users will be posted on this site.
3.7 EXIT FROM MOTA

This shuts down the MOTA environment. It is an alternative to the exit facility provided in theFile Menu.

4 ... FILE MENU: NEW ENGINE DATA FILE/EDIT EXISTING ENGINE DATA FILE

If you select Edit Existing Engine Data File a Windows dialogue box will open. Using the prompts on this menu, select the drive and folder in which the data file you wish to edit resides. Once selected, all valid data files will be listed. You may choose the file you wish to view or edit by double clicking it or by clicking it once and then clicking the "open" button on the dialogue box. You will then be presented with a screen entitled Data File Construction Utility, hereafter referred to as the DFCU screen. This same screen appears when you select the New Engine Data File from the File Menu.

The DFCU screen contains a number of buttons, which, when clicked will display screens on which you enter or change details of your engine. A red button indicates that the screen to which it refers has not been completely filled with data. A green button indicates that the screen to which it refers has been completed with valid data. The DFCU screen buttons may be clicked in any order at any time and the displayed screen then completed or modified. However, MOTA will not allow you to exit from any screen unless you have entered all of the required information properly. For many of the values to be entered the Data File Construction Utility has a built in validator which, when you try to move to the next screen, will advise you of the valid range of any variable whose value is unacceptable and then asks you to enter a valid value. You may open other Windows applications you may need to assist you in establishing data values, without having to close your MOTA screen. For example, you can make selections under the Tools Menu to assist in calculating information required by your current data file screen. If a required value cannot yet be decided, you may enter any reasonable value which satisfies the screen validator, and then return to update that screen later. This is a particularly important ploy as you cannot save a partially completed file. Once you start to build a new data file or to edit an old one, all of the buttons on the DFCU screen must be green, before you can save the file. This simply means that to save a file every screen to which a DFCU screen selection button refers must be completed with valid data even if this does not yet represent the final values of some of the data.

A Helpful Hint Rather than using the New Engine Data File selection it is often easier to create a new engine data file by choosing to edit an existing file which describes the same style of engine. Your new values are inserted as required and finally, when you save the file of new values, you can assign a new file name of your choice. This enables you to work through the Engine Data File without the need of inserting â€dummy†values and the entire construction of your new file can be completed over several edit sessions if desired.

Be aware that changing some numbers on one screen can make previously green buttons on the DFCU screen display red. That is, you will need to enter new data on the corresponding screens. For example, if you are editing a box muffler engine data file, and you change the muffler type option on the Engine Configuration screen from Box Muffler to a Single Piece Expansion Chamber, you will find that the relevant Expâ€n Chamber duct button on the DFCU screen will display red. You will then have to click this button during your editing session and enter the information required on the displayed screen.

When you have completed filling in an engine data file screen, you can exit this screen by clicking on the Next Screen button at the top of the screen. This returns you to the DFCU screen where you can now select another screen to construct or edit, or you can save the file if editing has been completed and no buttons display as red. If you click on the Exit to Main Menu button before you have saved your file you will be warned that to continue with the exit will result in the loss of the information you have entered from the keyboard and you are given the choice to save the file.

When you first enter the DFCU screen in New Engine Data File mode, you have only one column of red buttons available to you. This is because the Engine Configuration screen has not yet been filled in and so there is no information about the number of engine ducts and ports etc. Buttons pertaining to these appear only after you have filled in the engine configuration screen. Although you may fill in the screens corresponding to the initial set of red buttons in any order, the DFCU screen will not display the entire suite of buttons until the Engine Configuration screen has been filled in.

You can print the contents of any screen that you enter from the DFCU screen once you have filled it in. On each such screen there is a button with the caption Write to Print File. Clicking this button writes the content of the screen to a print file. When you select Next Screen and are returned to the DFCU screen, you will notice that the button Output Print File is no longer dull and is now active. Clicking this button will send the contents of the print file to the printer. If you want to print out the contents of a number of screens, do not click this button after exiting from each screen. Simply enter each screen for which you wish to print the content and click the Save to Print File button. The contents of each screen is added to the print file. Only when you have added all of the desired screen contents to the print file should you click the Output Print File button on the DFCU screen. The entire contents of the print file will then be sent to your printer. An additional button with the caption Print Entire Data File is active on the DFCU screen whenever all the data file screen selection buttons are displayed green. Clicking on this button will send the content of all data file screens to the printer in the order in which the screens are listed on the DFCU screen.

Clicking the Save the File button on the DFCU screen will display the normal Windows Save dialogue box. If you are editing a file, the name of your file will appear in the dialogue box, ready for you to OK the action. If you wish you may save to a different file name. It does not matter whether you enter the file name with or without the ".dat" extension, the extension is allocated correctly in either case. If you have just created a new file, you will need to enter a name for the file in the appropriate place on the dialogue box.

The remainder of this section (Section 4) describes each of the engine data file screens in detail.

The screens described in sections 4.1 †4.6 are completely independent of each other, whereas the remaining screens depend upon the content of the engine configuration screen.

Before proceeding it may be useful to define a number of abbreviations which are used within the discussion of some of the screens. These relate to the piston position and so also to the crank angle. Some of these abbreviations are also used on a number of the graphics output screens.

TDC: Top Dead Centre.

BDC: Bottom Dead Centre.

BTDC, ATDC: Before Top Dead Centre and After Top Dead Centre.

BBDC, ABDC: Before Bottom Dead Centre and After Bottom Dead Centre.

EPO: The crank angle (ATDC) at which the Exhaust Port starts to Open.

EPC: The crank angle (ATDC) at which the Exhaust Port becomes fully Closed.

4.1 ENGINE CONFIGURATION

4.1.1 Induction Type

The choice options are: Piston Port, Rotary Valve and Reed Valve induction types.

4.1.2 Exhaust System Type

The choice options are: Box Muffler, Integrated Muffler (Box Model), Single Piece Expansion Chamber and Integrated Muffler (Duct Model). A box muffler is a cavity, usually circular or rectangular in cross section, with an entry pipe from the engine and an exit pipe to the atmosphere.

Figure 1
Road Going Motorcycle Exhaust System comprising a Single Piece Expansion Chamber with Separate Muffler
 
Figure 2
Typical Go-kart Engine Expansion Chamber with Integrated Muffler
A Single Piece Expansion Chamber is shown in Figure 1. An Integrated Muffler is illustrated in Figure 2. If an exhaust system like that in Figure 1 is used without any type of muffler, you should choose the Single Piece Expansion Chamber muffler type. The Integrated Muffler (Box Model) and Integrated Muffler (Duct Model) refer to the same construction but a different mathematical approach is used to model the muffler in the MOTA engine simulator. In most cases the engine performance using either model is almost the same but there is evidence to suggest that the Duct Model will at times produce more realistic results.

The Integrated Muffler (Box Model) choice refers to the same model used in earlier versions of MOTA. If you use this option, you will be asked to enter only the muffler volume on the expansion chamber duct screen. No account is taken of the relative positions of the muffler, expansion chamber, and tail pipe.

If you make the Integrated Muffler (Duct Model) choice, you will need to fill in a screen for the muffler as well as a screen for the expansion chamber. In the former screen, you will be asked to provide geometrical information for each of the sections which make up the duct as well as the dimensions L1 and L2 illustrated in Figures 3a & 3b. An Integrated Muffler often comprises only one section whose diameter is the same as that of the largest diameter section of the expansion chamber. However, as shown in Figures 3a & 3b, if you are using the Duct Model for the muffler MOTA allows you to simulate a wide variety of constructions. If there is only a hole in the end of the muffler, you may specify this as a tail pipe of zero length and the appropriate diameter. This option is not available if you select the Box Model for the muffler.

Figure 3a                                                                                       Figure 3b
Typical Integrated Muffler constructions, showing some of the dimensions required for the Duct Model option
4.1.3 Number of Transfer Port Types

This is the number of different transfer port types. If some of the ports and their associated ducts are of the same dimensions and design, they account for just one transfer port type.

4.1.4 Induction Box

You should indicate whether or not your engine has a still air induction box.

4.2 BASIC ENGINE DIMENSIONS

Remember…. All dimensions of length are in millimetres and volumes are in cc.

4.2.1 Bore, Stroke, Connecting Rod Length

Self explanatory.

4.2.2 Cylinder Clearance Volume

The volume above the piston crown (with the spark plug installed) when the piston is at top dead centre (TDC).

4.2.3 Crankcase Clearance Volume

The crankcase volume below the piston crown when the piston is at bottom dead centre (BDC), excluding transfer duct volumes.

4.2.4 Gudgeon Pin Offset

If your engineâ€s gudgeon pin centre line coincides with that of the piston, enter "0.0". If the gudgeon pin centre line is displaced from the pistonâ€s centre line in the direction of the crank pin motion at TOP DEAD CENTRE, enter the amount of displacement (termed the offset) in millimetres. Enter an appropriate negative number if the displacement is in the direction opposite to the described crank pin motion.

4.3 IGNITION AND COMBUSTION DETAILS

4.3.1 The Combustion Efficiency

The efficiency with which an engine burns fuel. The combustion model used in MOTA is necessarily simple but quite effective. Experimental tests have shown this type of model to be widely applicable to a number of different engines when only the gross behaviour of the engine (i.e. its power output) is required. The value to use ranges from 0.8 for high performance engines with long exhaust port timings to about 0.85 for more conservatively timed engines.

4.3.2 The Burn Period

The total number of crankshaft degrees over which actual combustion takes place. Implicit in the use of the simple combustion model is a number of assumptions regarding the onset and cessation of burning in the cylinder. The burning period varies between about 60 degrees for a conservatively ported engine to about 50 degrees for a very high performance engine. For glo-plug ignition model aeroplane engines, the burn period is closer to 40 degrees.

4.3.3 Defining The Ignition Timing Advance Characteristic

MOTA allows different ignition timing (degrees advance) at different engine speeds. If you know these details, you can enter them at the known speeds and MOTA will assume a "straight-line" relationship for the ignition timing between these speeds. You should enter the number of engine speeds at which you know your engineâ€s ignition advance. If your engineâ€s ignition advance is a fixed value, enter the value "1" (i.e. 1 point only).

4.3.4 Ignition Advance

A table will be displayed with the number of columns equal to the number you entered in section 4.3.3 above. The first row of this table is completed by entering the engine speeds and the second row is then completed by entering the corresponding values of ignition advance. For the case where "1" was entered in section 4.3.3 above, only a single value of ignition advance will be required and this same value will be used at all engine speeds.

4.4 AMBIENT CONDITIONS

4.4.1 Ambient Pressure and Temperature

The units used to describe the prevailing air conditions are Pascals for pressure and degrees centigrade (C) for temperature. Conversion factors are displayed in case you work in the imperial units of pounds per square inch (psi) and degrees Fahrenheit (F).

4.4.2 Water Cooled Exhaust

Selecting this option invokes a model which assumes that the entire exhaust duct is surrounded by a jacket to allow the flow of cooling water. You are asked to enter the temperature (C) of this water.

4.5 FUEL AND SCAVENGE DETAILS

This screen collects details about the characteristics of your fuel and the scavenging characteristics of your engine.

4.5.1 Fuel Calorific Value (i.e. Fuel Heating Value)

A default value for 98 octane petrol is given. If you are using another fuel its calorific value in kcal/kg should be entered. Conversion factors you might need for this purpose are provided.

4.5.2 Throttle Setting (i.e. Throttle Area Ratio)

This is the fraction of the carburettor passage area open to flow. It varies from nearly zero when the throttle is closed to the value "1" when it is fully open. Note that the position of the throttle lever is not directly proportional to the flow area, i.e. if the throttle has a butterfly valve and the throttle lever can be rotated between 0 degrees (closed) and 90 degrees (fully open), the relationship between the throttle position and the flow area is:

Fraction of Carburettor Area Open to Flow = 1 - cosine(throttle lever rotation )
4.5.3 Number of Points Defining the Air/Fuel Ratio Characteristic

MOTA allows the use of different Air/Fuel Ratios at different engine speeds. If you know these values you can enter them with their engine speeds. To estimate the Air/Fuel Ratio at intermediate speeds MOTA assumes a "straight-line" relationship between adjacent tabulated values. You should enter the number of engine speeds at which you know your engineâ€s Air/Fuel Ratio. If you have only a single value for your engineâ€s Air/Fuel Ratio, enter the value "1" (i.e. 1 point only).

4.5.4 Air/Fuel Ratio(s)

A table will be displayed with the number of columns equal to the number you provided at section 4.5.3 above. The first row of this table is completed by entering the engine speeds and the second row is then completed by entering the corresponding Air/Fuel Ratios. However, for the case where a "1" was entered at section 4.5.3, only a single value is required and this same value will be used at all engine speeds. If you are unsure of the value(s) to use, the following is a very rough guide:

4.5.5 Scavenge Parameters

The MOTA scavenge model requires four scavenge parameters to be input. The following is a very broad set of guidelines to their values. You are encouraged to experiment with these values to obtain sensible results for your engine.

The first value represents the maximum fraction of incoming flow from the transfer port(s) to the cylinder which short circuits directly to the exhaust duct, without taking part in any mixing or displacement processes in the cylinder. It is assumed that maximum short circuiting occurs just after gas from the transfer port(s) starts to fill the cylinder and this decreases to zero as the cylinder fills. This maximum short circuit fraction usually lies between 0.0 and 0.2. It is never negative and rarely exceeds 0.4. For most high performance engines it is zero. For older engines and those of lower specific power output, a value of 0.2 and above is more appropriate.

The second value is the inflow displacement fraction which represents the maximum fraction of incoming flow which promotes displacement scavenging. Displacement scavenging is the bulk movement of exhaust gas out of the cylinder and into the exhaust port as a result of incoming gas from the transfer port(s). It is distinguished from perfect mixing scavenging by the fact that there is a definite interface between the fresh incoming gas and the exhaust gas and no mixing of the two gases occurs across this interface. It is usually a maximum at the beginning of the scavenging process. This parameter is never larger than 1.0 and rarely takes a value below 0.8.

The third scavenge value is the short circuit cutoff which is a measure of how far into the scavenge cycle the short circuiting flow described above falls to zero. This value must always be greater than zero. It is usually less than but close to 1.0 and generally should be set to 1.0. If short circuiting is to be constant throughout the scavenge process, as opposed to a uniform decrease, set this parameter to a large value like 10.0. If there is no short circuiting, set the first scavenge value to zero, and this parameter to any number greater than zero. The value zero must not be used.

The fourth scavenge value is the scavenge cutoff which is a measure of how far into the scavenge cycle displacement scavenging stops and perfect mixing controls the whole process. For high performance engines it generally lies between 0.7 and 1.0. For low performance engines, it is close to zero, but should never be set to zero. If there is to be no displacement scavenging, set the second scavenge value to zero and this parameter to any value greater than zero, this situation generally applying to low performing engines. For a constant fraction of displacement scavenging throughout the process, set this parameter to a large number like 10.0. Generally, however, this scavenge value should rarely be made greater than 1.0.

A summary of the recommendations for the values of these four parameters follows:

  1. Maximum Short Circuit Fraction (0.0 to 0.4): high performance 0.0; low performance 0.1 to 0.4.
  2. Inflow Displacement Fraction: 0.8 to 1.0
  3. Short Circuit Cutoff (must exceed 0.0): 1.0 recommended.
  4. Scavenge Cutoff: (must exceed 0.0): high performance 0.7 to 1.0; low performance 0.4 to 0.7.
4.6 SIMULATION RUN DETAILS

4.6.1 Output Units

All geometrical dimensions (lengths, areas and volumes) are output in metric units. However you may select whether you want the output of values such as those representing pressure, temperature, power and torque to be in metric or imperial units (i.e. kW kg C or hp lb F). In the graphical display of power and torque curves, your choice of units will be used as the default but it is then possible to toggle the display between the use of either choice of units. In the Engine Performance File the values of power and torque are provided in both metric and imperial units.

4.6.2 Comment for Performance File Header

This allows you to assign a unique identifier to your simulation, e.g. "KT100S low ports, GRZ test pipe, 65 mm step up".

It is recommended that you use a different identifier for different simulations of the same engine.

4.6.3 Pipe Step Factor

The flow of air, air/fuel mix and exhaust gases through an engine is modelled mathematically by the engine simulator in MOTA. This "fluid flow" model requires a user supplied value, known as the Pipe Step Factor for its operation. Throughout an engine simulation the Pipe Step Factor remains constant. It may be useful to provide some insight into the choice of value of this parameter.

To model the gas flow through an engine, MOTA divides each duct into a number of longitudinal elements. The length of these elements is controlled by the Pipe Step Factor. MOTA requires a minimum of four equispaced points along the shortest "passage section" in an engine to model the gas flow adequately. For example, suppose that this shortest length is a transfer duct whose length is 48 mm. For this engine a good choice of Pipe Step Factor is 48 divided by 4, that is 12. Generally this value will provide a good compromise between run time and accuracy.

Once values have been provided for all duct lengths the prompt for the Pipe Step Factor will include recommended upper and lower limits for its value. Generally, the upper limit will provide a suitable value but because the calculated values of the engine output will vary with this parameter, it may sometimes be necessary to use a value towards the minimum of the recommended range (see section 7, Running the Engine Simulator ).

As an example, for the Yamaha KT100S engine, the limits for the Pipe Step Factor suggested by MOTA are 15 and 3.7. In making your choice of value, bear in mind that halving the value of the Pipe Step Factor will increase the time for the simulator to complete its run on your computer by a factor of four.

4.6.4 Number of Engine Speeds

This is the number of engine speeds (i.e. number of different rpmâ€s) at which you wish to simulate the engine. The maximum number you can enter is 40. When selecting the number of engine speeds note that the Graphics Output File will provide values for the display of power and torque curves only where the simulation is performed over four or more engine speeds.

4.6.5 Engine Speed or Initial (lowest) Engine Speed

If you require an engine simulation at only a single speed, this is the relevant engine speed in rpm. For a multiple speed simulation, this is the initial and lowest engine speed in rpm.

4.6.6 Speed Increment

If the engine simulation is to be performed at more than one engine speed this is the engine speed interval in rpm at which the simulation will proceed.

4.6.7 Speed for Graphics Output

If the engine simulation is to be performed at more than one engine speed this is your choice of the engine speed at which output will be written to the Graphics Output File.

4.6.8 Maximum Number of Revolutions at Each Speed

MOTA simulates each engine revolution by modelling the engineâ€s behaviour mathematically at a large number of points throughout each revolution. As the number of completed revolutions increases, the output power should approach a steady value. Experiments show that for most engines an acceptably steady value is reached after about 30 revolutions. MOTA has an inbuilt mechanism for determining if it has converged successfully, so if you would like this mechanism to operate, you can input a large number of revolutions, for example 100, and let MOTA decide when it has converged at each speed. Where the engine simulation is for a single speed only, the value that you enter should not be less than 40. For a multi-speed simulation, the lower limit for the value you enter is 30 and, if the value you enter is less than 40, it will be ignored by MOTA on the first engine speed only and the value 40 substituted.

4.7 PISTON CONTROLLED PORT SCREENS

The second column of the Data File Construction Utility (DFCU) screen contains buttons linked to screens where the information describing your engineâ€s ports will be entered. In this section, the information to be provided for all piston ports is discussed.

Nearly every two-stroke engine contains at least one piston controlled port. Entry of data for this type of port is therefore common to all engine types. The accurate specification of these ports is crucial for the accurate prediction of an engineâ€s performance.

Port dimensions are provided in either one of two formats, that is as rectangular ports or as profiled ports (see Figures 4 & 5).
 

Figure 4
Rectangular Piston Port Configuration
A rectangular port is assumed to have parallel vertical sides and generally has radiused corners. Its specification is therefore quite straight forward, requiring the arc width, corner radii and the crankshaft angles after Top Dead Centre at which the port starts to open and at which it is fully open.

Figure 5
Measuring the Profile of an Irregular Port
A port should be specified as profiled only when the port shape is not rectangular, whether with or without radiused corners. In such case, instead of supplying a single port arc width and the corner radii, more information is required. This is the arc width of the port at various positions between the port starting to open, and becoming fully open. Such specification gives MOTA very accurate information about the relationship between the area of the port uncovered and the distance of the piston from top dead centre. You should use this option to describe any of your port shapes which deviate significantly from a rectangular shape with or without radiused corners. A construction for dividing the area of an irregular shaped port into a number of equispaced intervals is illustrated in Figure 5 and is described in section 4.7.3

The DFCU screen lists the number of different types of port. It is always assumed that there is only one type of inlet port duct and one type of exhaust port duct. You will have specified the number of different types of transfer port ducts on the engine configuration screen and, if this is more than one, they will be referred to as Transfer 1, Transfer 2, … on this screen. For each port type you are asked to enter the number of individual ports and say whether you will be providing profiled port data or not. If you answer yes to the profiled data question you are asked for the number of ordinates you wish to use to describe the changing width of the port. You can specify up to a maximum of 11 ordinates which corresponds to a maximum of 10 equispaced intervals.

If you have two identical ports, you are asked whether they constitute a bridged pair as distinct from two separate ports. A bridged pair consists of two individual ports of identical geometries, separated by a relatively narrow "bridge" and being fed by, or feeding into a single duct. MOTA needs this information so that it can calculate the total effective port area accurately. If you specify two or more unbridged ports, MOTA will assume that each port is fed by or feeds into a separate duct and that each of these ducts will be identical in geometry.

For engines with separate exhaust booster ports the area of these ports needs to be included with that of the main exhaust port(s). In the case of a port which is not profiled, this can be approximated by increasing the effective arc width of the relevant main port so that the total area of that port includes that of the booster ports. More accurately, the port may be considered as a profiled port so that the booster port areas can be included as part of the port profiling process (see section 4.7.3).

The exhaust port screen has a prompt for variable opening angle. This is provided for engines with variable exhaust port timing, but you must know the relationship between the port opening angle and the engine speed. If you select this option, you are asked for the number of engine speeds at which the exhaust port opening angle will be specified. You are then required to enter the appropriate number of engine speed/exhaust start open angle data pairs. A "straight-line" relationship is assumed between adjacent points.

In the following sections, the data which needs to be entered on the piston controlled port screens is discussed.

4.7.1 Data Common to Both Rectangular and Profiled Ports

For each piston controlled port, the crankshaft angle at which the piston starts to open the port, the crankshaft angle at which the port is fully open and the port attitude angles are requested. The message VARIABLE OPENING ANGLE  will appear in place of the port opening angle for an exhaust port with variable timing. The port attitude angles, namely the radial attitude angle and the axial attitude angle are defined in Figures 6 & 7 and 8a & 8b. If a port pair is defined as "bridged", the bridge width together with the top and bottom bridge radii must be provided for a rectangular port, but, for a profiled port, only the bridge width is required. By definition a bridged pair shares a common duct and the projection of the bridge into this duct is relatively small.

Figure 8a illustrates a section of a conventional rectangular bridged exhaust port. OM represents a radius of the cylinder drawn to the mid-point M of the right hand port arc width. The radial attitude angle Ar is the angle made by OM and a line through M which is parallel to the port side wall close to the cylinder.

An option provided on the Tools Menu can be used to calculate the value of the radial attitude angle of a bridged port pair. You will have to provide the values of the port arc width XY and the bridge arc width YZ.

An alternative construction is illustrated in Figure 8b. In this case the port wall close to the cylinder is parallel to the radius OM and experience suggests that a zero radial attitude angle should be used for such a port configuration.

For exhaust ports with variable opening timing, a table will be displayed for you to input the opening angle of the exhaust port at each of the engine speeds at which you have this information.

4.7.2 Rectangular Ports

This information is required when the port shape is rectangular with radiused corners (see Figure 4 ). In addition to the values described above, the port arc width and the top and bottom corner radii must also be provided. Note that the value to be used for the port width is the arc width. A facility to convert between chord width and arc width values is provided by clicking Tools on the Main MOTA Menu and then making the appropriate selection from the Tools Menu.

Figure 6
"Ax" is the Axial Attitude Angle
 
Figure 7
"Ar" is the Radial Attitude Angle


Figure 8a                                                                                         Figure 8b

Radial Attitude Angle for Alternative Bridged Exhaust Port Geometries

4.7.3 Profiled Ports

If you have chosen the profiled port option and have entered the number of ordinates in the profile data, you will need to fill in a table of ordinate port arc widths and the corresponding crankshaft angles.

Irregularly shaped ports should always be described by such a sequence of arc widths (ordinates) measured over a set of equispaced intervals spanning the full height of the port. These ordinates should be submitted to the engine data file in the same order as that described by the port opening sequence. In the case of a profiled inlet port, particular care should be taken to submit the ordinates in the correct sequence of opening order, that is from the bottom of the cylinder upwards.

A construction which divides one of the ports of an irregular shaped bridged port pair into six equispaced intervals, providing seven ordinates, is illustrated in Figure 5. A pair of compasses, a pair of dividers, a rule, a set square and a fine pen or sharp pencil are minimum requirements for the performance of this work. Generally a suitable port outline can be obtained from a "rubbing" of the cylinder wall.

MOTA treats each division between adjacent pairs of ordinates as a trapezium and with this in mind it will in some cases be appropriate to adjust the lengths of the first and/or last ordinates in order to provide a realistic port area. Again note that ordinates representing arc width values should be provided. The construction illustrated in Figure 5 is completed as follows:

First draw parallel base and top lines at either extreme of the port height. Next, using a point O on the base line as centre, draw an arc which intersects the top line at point A. The radius of this arc will exceed the port height and in particular, it should be easily divided into the required number of equal intervals, in this case six. Now mark the equispaced divisions of OA and finally draw the ordinates L1 to L5 as a sequence of lines parallel to the base and top lines. The lengths L0 to L6 are best measured using a pair of dividers. Notice that the values of L0 and L6 have been adjusted in an attempt to provide realistic representations of the areas of the two extreme sections of the construction.

4.8 INLET ROTARY VALVE DETAILS

This screen is provided only if you selected the rotary valve inlet option on the Engine Configuration screen. Prompts on this screen ask you to enter the rotary valve data, a schematic of which is shown in Figure 9.

4.8.1 Number of Rotary Valves

You may have one duct or two identical ducts feeding one cylinder. The latter configuration is assumed to require two identical rotary valves. Enter the number of rotary valves here.

4.8.2 Opening Angle

This is the crankshaft angle before top dead centre at which the port starts to open.

Figure 9
Rotary Valve Configuration
4.8.3 Closing Angle
 
 

This is the crankshaft angle after top dead centre at which the port becomes completely closed.

4.8.4 Radius to Bottom of Port

The port is assumed to be radially symmetric with respect to the crankshaft centre as in Figure 9. Any other geometry will have to be approximated within these bounds. The radius to the bottom of the port is shown as Rv in Figure 9.

4.8.5 Port Height

This is shown in Figure 9.

4.8.6 Angular Port Width

This is the angle A in Figure 9. It is assumed that the port sides will lie along radial lines from the crankshaft centre, possibly with small radius corners. Any other geometry will have to be modified to fit this specification.

4.8.7 Corner Radii

This is shown in Figure 9. All corner radii are assumed equal.

4.9 INLET REED VALVE DETAILS

This screen is provided only if you select the reed valve engine option on the Engine Configuration screen. Prompts on this screen ask you to enter data for the reed valve. A typical reed valve is illustrated in Figure 10.

4.9.1 Number of Reed Valves

You may have one duct or two identical ducts feeding one cylinder. The latter configuration is assumed to require two identical reed valves. Enter the number of reed valves here.

4.9.2 Valve Block

4.9.3 Uniform/Stepped Petal Thickness (Single and Compound Petals)

Generally a reed block port is controlled by a single petal. However, in some cases two petals are clamped together with the length of the outer petal less than that of the inner petal. Such a configuration is referred to as a compound petal. MOTA requires you to specify the type of petal, single or compound, contained in your reed valve.

4.9.4 Petals

Values of Youngâ€s modulus and the density for a few common reed petal materials are provided in a table on this screen.

For compound petals, where one petal overlaps the other over part of the unclamped length, prompts appear for the combined thickness of both petals, and the unclamped length of the shorter petal.

Figure 10
Reed Valve Details
4.10 DUCT SCREENS

The third column of the DFCU screen contains buttons which are linked to screens which, when completed, will contain all necessary information about your engineâ€s ducts. Throughout this manual, "Duct" is the general term used to refer to any passage down which gas flows, for example, the carburettor, inlet manifold, exhaust pipe and the passages leading from the crankcase to the transfer port(s). MOTA assumes that each duct comprises a sequence of linearly tapered sections. Figures 1 & 2 show typical duct sections for two different exhaust systems. Discontinuities in cross-section area at section junctions are allowed. Typically this may occur in an exhaust duct if the cross sections of the barrel section and the engine pipe are not perfectly matched at the barrel flange. Inlet and transfer ducts can also have more than one section. The information required for all ducts is discussed in the following sections.

4.10.1 Data Common to All Ducts

The number of sections in each duct. The inlet duct must have a minimum of two sections (even if each has the same cross-section area), the transfer duct and muffler duct must each have a minimum of one section and the expansion chamber duct must have a minimum of 4 sections.

In counting the number of inlet duct sections, do not include:

MOTA adds an extra section automatically for each of these last sections and prompts you for the relevant dimensions.

Because they are so short, most transfer ducts are modelled as single section ducts.

Figures 1 & 2 show how sections are determined and numbered for expansion chambers. Notice that neither the part of the exhaust port duct which is contained in the cylinder barrel nor the tail pipe (if one is present) is considered to be an expansion chamber section for the purpose of data entry. The details of these duct sections are entered separately.

Section numbering for each duct begins at the "inflow" end of the duct, that is, at the end of the duct into which fluid (air, air fuel mix or the engine exhaust) normally flows. MOTA assumes that for the inlet duct, the first section starts immediately after any bellmouth present, so your first section measurements should not refer to any bellmouth.

Once the number of duct sections is entered, a table is displayed with spaces set for the entry of the inlet diameter, the outlet diameter and the length of each section. The table is scrollable if the duct comprises more than four sections. For some transfer ducts you may not be asked to enter the outlet diameter of the final section. This will be the case if you selected the option "Smooth Exit" for the transfer duct. This option forces MOTA to set the area of the last transfer duct section outlet to be the same as the corresponding transfer port effective area, thus fixing its equivalent diameter.

Figure 11
Physical Layout of Transfer Duct Geometry
The correct identification of the property smooth entry to or smooth exit from the duct is important when completing most of the duct screens. As an illustration, consider Figure 11 which shows a schematic of a single section transfer duct. The inlet diameter (D1, crankcase end) is usually very close in value to the outlet diameter (D2, transfer port end), so in many cases, an equivalent parallel duct of constant circular cross section can be assumed. The dimensions D1 and D2 are the diameters of a circle of the same area as the port openings. For D1 this may be most easily determined by using a rubbing of the inlet to the duct onto 1 mm square graph paper. Counting the squares gives an area which can be converted to a circle whose diameter is D1 using the formula:

D1 = O (1.2732 x Area)

Notice that if the port is close to rectangular in shape with radiused corners the Tools Menu provides an option for evaluating the diameter of the circular section having the same area.

If the end of a duct at the join with a piston port flares out markedly before it mates with the port, the duct should be marked as having a non smooth entry or exit, as appropriate, on the relevant duct screen. Figure 11 depicts such a situation for a single section transfer duct. In addition to the dimensions D1 and LT, you will be asked for the equivalent diameter (D2) immediately before the flaring occurs. For a single section transfer duct the corresponding length is the length of the transfer duct along its centreline from its beginning in the crankcase to its end at the piston face. The extension of this procedure to deal with inlet and exhaust ducts and with multiple section ducts follows naturally. The need to divide a transfer duct into more than one section should rarely arise.

4.10.2 Inlet and Induction Ducts

The carburettor, any reed or rotary valve present, and associated ducting into the cylinder head or crankcase forms the inlet duct. It is assumed that the inlet duct consists of:

On the Engine Configuration screen, you indicate whether or not your engine contains an induction duct (in addition to the normal inlet duct). Such a duct leads from the atmosphere to a still air box from which the normal inlet duct feeds. If your engine has an induction duct, you need to indicate on the Induction Duct screen whether or not this duct contains an entry bellmouth. You will also be asked to enter the volume of the induction (still air) box on this same screen.

For all types of induction, you must specify whether or not there is an entry bellmouth. For piston port induction, there is an additional option box labelled smooth exit. A smooth exit prevails when the outlet area of the section of inlet duct in the cylinder barrel is the same as the inlet port area, that is, when the duct section is not flared into the port. For reed valve and rotary valve induction, it is assumed that a smooth exit occurs and so the option box for this property is not displayed.

There is also a frame which is titled Cylinder Barrel Duct Section for piston ported induction, Inlet Disk Valve Cover Duct Section for rotary valve engines, and Inlet Reed Valve Block Duct Section for reed valve induction. Information regarding this last inlet duct section, referred to under the second bullet point above, must be entered here. For each type of induction, this comprises the duct diameter at the mounting flange and the length of the duct from its entry to the piston face (piston port) or to the reed valve petal tips (reed valve) or to the rotary valve disk face (rotary valve). In addition, for a piston port controlled engine, if you have specified a non-smooth exit into the inlet port the diameter at port input box will be active. This means that due to flaring, the duct area adjacent to the inlet port is different from that of the inlet port itself, and so you need to specify the equivalent diameter of the duct close to the port but immediately before the flaring.
 
 
 

Figure 12
A Typical Inlet Duct with Reed Valve
Notes on Figure 12 :

For a rotary valve inlet port, the final section is the section between the mounting flange and the rotary valve disc face.

For a piston ported engine, the final section is the section between the mounting flange on the cylinder barrel and the piston face.

The assembly to the left of the mounting flange contains the Carburettor which may be fitted with a bellmouth at its intake end. Another duct section may exist between the carburettor and the barrel (piston port induction) or the crankcase flange (reed valve or rotary valve induction).

For the configuration illustrated, the number of sections to be entered on the Inlet Duct screen is "3" and not "4" because details of the section adjacent to the reed (or rotary or piston) port are entered separately on this screen as previously described.

4.10.3 Exhaust Duct

The smooth entry option required for this screen has the same meaning as the smooth exit option on the piston port induction Inlet Duct screen and on the Transfer Duct screen(s). Your choice will determine whether or not the Diameter at Port input box within the Exhaust Duct Section in Cylinder Barrel frame needs to be entered (see Figure 13). If the box is not dulled, you will need to enter the equivalent diameter of the exhaust duct at its entry from the cylinder but beyond the flaring at the port in the cylinder barrel. You will always need to enter the diameter of this section at the cylinder barrel flange and the length of the section between this flange and the piston face. Although the MOTA engine simulator treats this section as an exhaust duct section, it must not be included in the Number of Duct Sections entered lower down the screen.

If your engine is of the Box Muffler type, there will be only one section (by default) in the exhaust duct and you will not be asked for the number of sections in the exhaust system. There will also be a tail pipe diameter and length to be provided, as well as a muffler volume.

If you have chosen to model your engine with either an Integrated Muffler (Box or Duct Model) or with a Single Piece Expansion Chamber, you will be asked for the tail pipe length and diameter in addition to the details common to all exhaust systems. The muffler volume will also be required for the Integrated Muffler (Box Model).

If you have chosen to model your engine with an Integrated Muffler (Duct Model) (as distinct from an Integrated Muffler (Box Model) †see section 4.1.2), the exhaust duct description will be entered over two screens. The corresponding buttons on the DFCU screen are labelled Expansion Chamber and Duct Muffler. The Expansion Chamber screen will require the same details as for a Single Piece Expansion Chamber engine, less the tail pipe details. The Duct Muffler screen requires details of the muffler can and the protruding tail pipe. The muffler can may comprise several conical sections, just like the expansion chamber, so the relevant prompts are identical, as are those for the tail pipe which is assumed to pass through the rear face of the muffler can. The screen contains a frame labelled Protrusions within the Muffler which provides two prompts. The first prompt is Expansion Chamber Length Inside the Muffler which relates to dimension L1 in Figures 3a & 3b which is the protrusion of the expansion chamber, plus any stinger length, into the first section of the muffler can. The second prompt is Tailpipe Length inside Muffler which relates to dimension L2 in Figures 3a & 3b. If the tail pipe length is zero, the corresponding input box will be inoperative (dull), signifying just a hole in the rear of the muffler can, equal in diameter to the specified tail pipe diameter. However, if the tail pipe length is non-zero you are required to enter how far it protrudes into the muffler can.

Figure 13
An Exhaust Barrel Section
When an Expansion Chamber Duct screen is displayed, the additional button Display Profile appears on the screen. Clicking this button displays a schematic of the expansion chamber and a table of dimensions which includes the cone angle and volume of each section. The content of this screen can be sent to the printer.

5 ... FILE MENU: VIEW AN ENGINE PERFORMANCE FILE

The Engine Performance File lists your engineâ€s characteristics, as entered in the engine data file construction screens, as well as a number of performance measures for your engine. If this menu item is selected, you are asked to select a valid file (such files will have the extension ".per" and are generated by the MOTA engine simulator). The file selection procedure is identical to that for selecting a data file to edit (section 4), except that only Engine Performance Files will be listed.

The file commences with a header which provides:

Engine specification and performance details provided by the file are described below.

5.1 DEFINITIONS OF ENGINE PERFORMANCE MEASURES

5.1.1 Compression Ratios

5.1.1.1 Cylinder Compression Ratio

In MOTA this is defined on the basis of the trapped gas volume rather than the swept volume.

Let V1 be the trapped gas volume above the piston at the point when the exhaust port closes.

      V2 be the cylinder clearance volume, that is, the volume above the piston at TDC.

Cylinder Compression Ratio = V1/V2

5.1.1.2 Crankcase Compression Ratio

Let V1 be the swept volume of the cylinder.

      V2 be the crankcase clearance volume, that is, the volume below the piston crown at BDC excluding the volume of the transfer duct(s).

Crankcase Compression Ratio = (V1 + V2)/V2

5.1.2 Power Output and Torque

By power output is meant the brake power output of the engine. It is the power which is delivered at the crankshaft and represents the power available for the performance of useful work. The units used by MOTA are kW (metric) and hp (imperial). Practically, values of engine power over a range of engine speeds are obtained by running the engine on a dynamometer.

By torque is meant the turning effect provided at the engine crankshaft. The units used are Nm (metric) and ft lbf (imperial).

Conversion factors between metric and imperial units are provided at the end of this manual.

The relation between brake power output and torque is

Brake Power Output = C1 x N x T
where N is the engine speed (rpm), T is the torque and C1 is a constant whose value depends upon the units. For the metric units of kW and Nm the value is C1 = Pi/30000 and for the imperial units of hp and ft lbf the value is C1 = 2 x Pi/33000.

5.1.3 Mean Effective Pressures

In MOTA, the units used for the display of pressure is atmospheres (atm). Conversion factors to other metric and imperial measures of pressure are provided at the end of this manual.

5.1.3.1 Brake Mean Effective Pressure (BMEP)

The brake mean effective pressure (bmep) is defined in terms of the brake power output.

Let V be the swept volume of the cylinder.

      N be the engine speed (rpm).

Brake Power Output = C2 x V x N x BMEP where C2 is a constant whose value depends upon the units.

Using a known value of Brake Power Output the corresponding value of Brake Mean Effective Pressure (BMEP) may be calculated as

BMEP = Brake Power Output/(C2 x V x N).
For metric units with V in c.c., BMEP in Pascals and the brake power output in kW the constant value is C2 = 1/60000 and for imperial units with V in cubic inches, BMEP in psi and the brake power output in hp the constant value is C2 = 1/396000.

5.1.3.2 Pumping Mean Effective Pressure (PMEP)

In a two-stroke engine, power is expended in compressing the air/fuel mix in the crankcase and delivering this mix to the engine cylinder. This is referred to as the pumping power loss.

Let P1 be the pressure in the crankcase at some point between TDC and BDC in the engine cycle.

      P2 be the pressure in the crankcase at the same piston position but between BDC and TDC.

The PMEP is the average of the difference (P1 - P2) over the entire engine cycle. This value can be determined using sensitive equipment. The associated loss of power may then be calculated as

Pumping Power Loss = C2 x V x N x PMEP
where C2 is a constant whose value depends upon the units, V is the swept volume of the cylinder and N is the engine speed (rpm).

Values of C2 are given in section 5.1.3.1.

5.1.3.3 Friction Mean Effective Pressure (FMEP)

Friction losses occur in the engine bearings and during the motion of the piston in the cylinder. This friction power loss represents a reduction in the power available for the performance of useful work.

Let V be the swept volume of the cylinder.

      N be the engine speed (rpm).

Friction Power Loss = C2 x V x N x FMEP where C2 is a constant whose value depends upon the units.

Values of C2 are given in section 5.1.3.1.

Notice that the FMEP is calculated by subtraction as FMEP = IMEP - BMEP - PMEP and the IMEP is defined in the following section.

5.1.3.4 Indicated Mean Effective Pressure (IMEP)

The power developed in the engine cylinder is referred to as the indicated power output.

Let P1 be the pressure in the cylinder at some point between TDC and BDC in the engine cycle.

      P2 be the pressure in the cylinder at the same piston position but between BDC and TDC.

Now average the difference (P1 - P2) over the entire engine cycle.

This average value is the Indicated Mean Effective Pressure (IMEP). It can be determined using sensitive equipment. The associated indicated power output may then be calculated as

Indicated Power Output = C2 x V x N x IMEP
where V is the swept volume of the cylinder, N is the engine speed (rpm) and C2 is a constant whose value depends upon the units.

Values of C2 are given in section 5.1.3.1.

By definition: IMEP = BMEP + PMEP + FMEP

and: Indicated Power Output = Brake Power Output + Pumping Power Loss + Friction Power Loss.

5.1.4 Brake Specific Fuel Consumption (BSFC)

This is the amount of fuel consumed by the engine in an hour per unit of brake power output. The engine speed should be given when quoting this value. The respective metric and imperial units used in MOTA are kg/kwh (kg per kilowatt hour) and lb/hph (lb per horsepower hour).

5.1.5 Duct Flow Ratios

In MOTA, the inlet end of a duct is defined as the end of the duct into which the gas passing through the duct generally flows. For example, the inlet end of a transfer duct is the end in the crankcase. Sometimes there is a small reversal of the flow at this end but most of the gas flowing in this duct enters from the crankcase. The outlet end of a duct is defined similarly. For example, the outlet end of a transfer duct is the end at the cylinder which is controlled by the piston.

During steady operation of an engine, for each duct, the net amount of gas flowing into the inlet end of the duct should be identical to that flowing from the outlet end of the duct over an engine cycle. The property of a mathematical model of gas flow through a duct to reproduce this equality of inflow and outflow is known as mass conservation. There are several different models of gas flow which can be used in an engine simulation. Some of these models are unable to conserve mass flow and can produce a difference in duct inflow and outflow of over 60%. The accuracy of engine performance predictions using such models is questionable when the difference in the predicted inflow and outflow of any duct exceeds 10%.

The duct flow model in MOTA is one of a new generation of models which guarantees mass conservation. However, in order to achieve this conservation the model must be run for a sufficient number of revolutions (see section 4.6.8 of this manual). If insufficient revolutions are performed during the simulation, mass conservation will not be achieved. In practice, mass conservation errors below 0.1% should be obtained in MOTA simulation runs. This error is negligible and is due to the fact that most simulations are run for only 40-50 revolutions at each speed. The higher the number of revolutions, the better the mass conservation, but the longer the simulation will take to run.

In MOTA, the flow ratio at a particular duct end is defined as follows:

let Mf be the net gas mass flow into or out of the duct per cycle, according to the end of the duct under consideration.

      Mr be the mass of air occupying the cylinder swept volume at atmospheric conditions.

Duct End Flow Ratio = Mf/Mr

The delivery and exhaust flow ratios are provided in the MOTA output so that the user can check that sufficient revolutions have been completed to achieve mass conservation throughout the engine. The delivery flow ratio is the flow ratio for gas flowing from the inlet duct into the engine. The exhaust flow ratio is the flow ratio for gas flowing out of the cylinder into the exhaust duct. In most cases these should differ by less than 0.1%. However, if they differ by more than 2%, the simulation should be re-run for a higher number of revolutions. In some cases, differences of 2-3% may occur at very low speeds in high performance engines, but decrease to normal levels after the first one or two speeds. In such cases, the simulations do not need to be re-run. The differences at these low speeds are caused by other factors and do not affect the integrity of the model.

5.1.6 Scavenge Ratios

5.1.6.1 Mass Based Scavenge Ratio

In most cases this value is identical to (the sum of) the flow ratio(s) at the outlet end(s) of the transfer duct(s). However, it differs in definition by the fact that it refers only to pure air/fuel mix entering the engine, instead of all gas. For example, if there were significant flow of exhaust gas back down the transfer duct(s) before normal direction flow was restored, the scavenge ratio only takes account of the net air/fuel part of the mixture which flows into the engine cylinder.

Let Mf be the net mass of the air/fuel mix flowing from the transfer duct(s) into the engine per cycle.

      Mc be the mass of air occupying the cylinder swept volume at atmospheric conditions.

Mass Based Scavenge Ratio = Mf/Mc

5.1.6.2 Volume Based Scavenge Ratio

Let Vf be the total volume of the air/fuel mix flowing from the transfer duct(s) into the engine per cycle.

      Vc be the swept volume of the cylinder.

Volume Based Scavenge Ratio = Vf/Vc

5.1.7 Efficiencies Related to the Scavenge Process

In an internal combustion engine, scavenging is the process which replaces the burnt gases in the cylinder with a fresh charge of air or air/fuel mix.

In a two-stroke engine with piston controlled transfer and exhaust ports these ports are simultaneously open during a major part of the engine cycle. During this time there will be some mixing of fresh air/fuel mix with burnt gas and some loss of fresh air/fuel mix to the exhaust system. The ideal is perfect scavenging where all burnt gas flows through the exhaust port with no mixing of burnt gases with the fresh air/fuel mix and where no fresh air/fuel mix is lost to the exhaust.

5.1.7.1 Scavenge Efficiency

A number of slightly different definitions of Scavenge Efficiency exists. That used by MOTA is based upon the contents, by mass, of the cylinder at the point when the exhaust port becomes fully closed. At this point some burnt gas remains in the cylinder and mixes with the new charge of air/fuel mix.

Let M1 be the mass of the fresh air/fuel mix in the cylinder when the exhaust port closes.

      M2 be the mass of burnt gas remaining in the cylinder when the exhaust port closes.

Scavenge Efficiency = M1/(M1 + M2)

The smaller the value of M2 relative to M1 the greater the amount of fuel which is available for combustion and the higher the power output of the engine.

5.1.7.2 Trapping Efficiency

The Trapping Efficiency provides a measure of the unburnt air/fuel mix which is lost to the exhaust during the scavenging process and before the exhaust port becomes fully closed. Notice that the Scavenge Efficiency is not influenced by this loss of unburnt air/fuel mix.

Let M1 be the mass of the fresh air/fuel mix in the cylinder when the exhaust port closes.

      M2 be the mass of fresh air/fuel mix entering the engine during one complete cycle.

Trapping Efficiency = M1/M2

The smaller the loss (M2 - M1) of unburnt air/fuel mix to the exhaust per cycle the better the utilisation of fuel by the engine.

5.1.7.3 Charging Efficiency

The flow of gas within the engine is a dynamic process subject to changing pressures, velocities and temperatures. In a well designed engine the exhaust gas will be extracted rapidly from the cylinder and the fresh charge from the crankcase will flow rapidly into the cylinder. There will be little mixing of the burnt gases with the fresh charge and the exhaust gas flow dynamics will prevent excessive loss of fresh charge into the exhaust system. The better the gas flow dynamics the greater the quantity of fresh air/fuel mix which charges the cylinder per engine cycle. The Charging Efficiency provides a measure of the effectiveness of this process.

Let M1 be the mass of fresh air/fuel mix in the cylinder when the exhaust port closes.

      M2 be the mass of air that would occupy the swept volume of the engine cylinder at atmospheric conditions.

Charging Efficiency = M1/M2

5.1.8 Percentage Energy Loss in the Exhaust System

Whilst the amount of gas entering an engine duct over an engine cycle must be equal to that leaving it, the same is not true of the energy content (heat energy plus kinetic energy) of the gas. As the gas travels through a duct, friction occurs (when kinetic energy is converted to heat energy) and heat energy is lost through the surface of the duct to the atmosphere and to any other cooling medium (such as water) which is present.

Let E1 be the total energy of the gases passing from the engine cylinder to the exhaust system per cycle.

      E2 be the total energy of the exhaust gases passing to the atmosphere per cycle.

The Percentage Energy Loss in the Exhaust System = 100 x (E1-E2)/E1

A value of 2-3% for this loss is considered normal and acceptable. A value exceeding 10% suggests that some form of insulation or redesign of the exhaust system should be considered.

5.2 SIMULATION PARAMETERS

5.3 ENGINE GEOMETRY 5.4 FUEL PARAMETERS 5.5 COMBUSTION PARAMETERS 5.6 AMBIENT CONDITIONS 5.7 PISTON PORT DIMENSIONS

The relevant data for each piston port type is displayed under this heading. First, the port name, the number of ports, the port angular, arc and chord widths, the port height and for non profiled ports the top and bottom corner radii are displayed. Also, for a port pair, the bridge status is displayed. Next, for any bridged port pair, the bridge width, the radii at the top and bottom of the bridge and the effective maximum chord width are displayed. Finally, for each port type, the total area and the two attitude angles are displayed.

5.8 PISTON PORT PROFILE DETAILS

Output under this heading is provided only where at least one port type is defined by means of a sequence of equispaced arc width ordinates. For each such port the sequence of arc width ordinates are tabulated together with the corresponding chord widths and values of total cumulative port area.

5.9 PISTON PORT TIMING DETAILS

The crank angle and the corresponding piston displacement from top dead centre at which each port starts to open and is fully open. If your engine contains a variably timed exhaust port, the timing details are listed.

5.10 ROTARY VALVE PORT DATA (if applicable)

5.11 REED VALVE PORT DATA (if applicable)

5.11.1 Block Dimensions

5.11.2 Petal Dimensions 5.11.3 Port Dimensions 5.12 DUCT DIMENSIONS

The number, length, inlet and outlet diameters, inlet and outlet cross section areas of each duct section in the engine. Note that each duct section is assumed to be uniformly tapered between its inlet and outlet diameters. The cone angles and volumes are also listed for each expansion chamber section if your engine is of this type.

5.13 ENGINE PERFORMANCE INDICATORS

The performance indicators which are listed below are provided at each simulation speed.

6 ... FILE MENU: DISPLAY ENGINE GRAPHICAL INFORMATION

When you select this option you will be asked to choose the name of a Graphics Output File (these have the extension ".gph") in exactly the same way that you chose a file in the Edit Existing Engine Data File and View an Engine Performance File options. The graphical information will then be read into MOTA from the selected file. Down the left hand side of your MOTA screen a number of buttons will be displayed. Clicking these buttons will display graphically different aspects of your engineâ€s performance. A displayed graph can be sent to your printer by clicking either the Clr Prt button if you have a colour printer or the B/w Prt button otherwise. However, in the case of animated waves, you have to stop the animation by clicking the Wave Stop button before the two print buttons are made operative. If you have a colour printer the colour print option is particularly useful when printing power and torque curves where one or more overlays (see section 6.1) have been selected. This is because the colour coding of the different curves is preserved.

If the simulation run which produced the Graphics Output File involved four or more engine speeds the engine power and torque achieved at these speeds can be exhibited graphically and in such case these power curves are immediately displayed as the default.

The graphics display button functions are as follows:

6.1 POWER/TORQUE (if applicable)

If you run your engine simulations at four or more engine speeds, this option button is made operative. It allows you to display graphs of engine power and torque plotted against engine speed. The solid lines on the graph represent engine power and the dotted lines represent engine torque. When you select this option and also, when it is initially shown as the default display (see above), four frames are added beneath the buttons at the left hand side of the MOTA environment.

The first Units frame allows you to change the units in which the graphs are displayed, that is either kW for the power and nm for the torque or hp for the power and ft lbf for the torque. The default display will correspond with the Output Units selection made on the Run Parameters screen when the corresponding Engine Data File was prepared.

The second Display frame allows you to choose whether to display both power and torque, or either one singly. The single display of torque curve(s) uses solid and not dotted lines.

The third Overlay frame allows you to add and remove overlays from the graph. Adding an overlay displays a Windows dialogue box asking you to select a file for overlaying. Although only Graphics Output Files (.gph) are displayed initially, clicking the file type display on the dialogue box allows you to select Power Files (.pow) as well (the construction and use of Power Files is described in section 9).

The fourth Scales frame allows you to change the scales which were chosen by MOTA and, once such a change has been made you can make further changes or choose to restore the default scales to the display.

You may select up to 5 overlay files. You may remove an overlay at any time by clicking the Remove button. On doing this, a list of overlaid files is displayed and clicking a member of the list removes it from the display.

Note: When you add an overlay file the current scales choice is not changed, even if this means that the newly displayed curve is clipped. It has been found that this property is most useful to the engine tuner/designer when comparing the performance of various engine designs. Of course you can change the scales at any time by clicking on Change in the Scales frame and then entering replacement scale limits of your choice. Alternatively you may elect to use the scales provided as the default by MOTA by clicking on Restore Default in the Scales frame, the scales so provided will display each of the selected curves in full.

Also displayed within the plotting area of the power and torque curves is a black vertical bar which extends over the entire height of this area. Where the bar intersects each curve a horizontal cursor is drawn and, to the right of the plotting area, the corresponding power and torque values and the engine speed are displayed. The position of this cursor bar can be controlled by the mouse left button, enabling the display of power and torque values at any chosen engine speed within the plotting range. Both click to move and drag mode control of the cursor bar are provided.

6.2 WAVES

Selecting this option allows you to view the propagation of Pressure, Velocity, Temperature, Density and Purity waves in each engine duct. The display is animated in the sense that it displays the change in the relationship between the chosen variable and the distance down the selected duct several hundreds of times throughout an engine revolution. It repeats this display continuously for as long you wish. You may alter the animation speed by changing the frame speed setting of the horizontal scroll bar displayed towards the bottom of the screen.

When the waves button is clicked, a frame listing of all of the engine ducts appears. Selecting a duct displays another frame listing the various types of wave (pressure, velocity, etc). Selecting a wave type results in the animation described above. You can change the wave type simply by selecting another one. If you change the duct, the wave type frame initialises itself and you must again select a wave type to display the animation.

To print an animation at any one time, click on Wave Stop to freeze the animation and then click on Clr Prt if you have a colour printer or click on B/w Prt otherwise. After the still frame has been printed, you can click on Wave Start and the animation will continue.

Because of the huge range and quality of VGA video adapters, it is impossible to ensure that the animated graphics will produce smooth and continuous images on all screens. When images flicker and jump, adjusting the frame speedcontrol on the display screen should rectify the problem.

6.3 CYLINDER-EXHAUST PRESSURE DIFFERENCE

The difference between the pressure in the cylinder and the pressure at the cylinder end of the exhaust duct determines whether gas flows from the cylinder into the exhaust duct or into the cylinder from the exhaust duct. In designing a two-stroke engine, you want to extract the combustion gases as quickly as possible and then plug the exhaust port with high pressure exhaust gas to prevent the in-coming fresh charge from the transfer port flowing out through the exhaust port before it is ignited. This option allows you to examine the relationship between the cylinder-exhaust pressure difference and crankshaft angle, to determine how well the exhaust extraction/fresh charge plugging process is working.

6.4 INDUCTION REED VALVE PETAL LIFT (if applicable)

If your engine is fitted with an induction reed valve, this will display the variation of the reed petal tip lift with crankshaft angle.

6.5 SCAVENGE RATIO

At any crankshaft angle, this is the sum of the delivery ratios (see section 6.6) at the outlet end of each transfer duct.

6.6 DELIVERY RATIOS

Over one complete engine revolution, the amount of gas flowing into each engine duct should be identical to that flowing out of the duct at its other end. However, within an engine cycle, the variation with crankshaft angle of the gas flow into one end of a duct will be different from the variation of gas flow out of the duct at the other end. The delivery ratio at any point in time is just the mass of gas which has flowed through a particular duct end from the start of the engine revolution, divided by the mass of gas in the cylinder at ambient conditions with the piston at bottom dead centre.

Selecting this item provides a sub-menu list of the engine ducts. By clicking on a duct name you obtain the display of both the inflow and outflow delivery ratio curves for the selected duct.

Note that for an engine with multiple transfer duct types, each duct will contribute to the total flow into the engine cylinder, so the Scavenge Ratio (see section 6.5) at any crankshaft angle will be the sum of the delivery ratios at the outflow end of each transfer duct.

6.7 BOX CHARACTERISTICS

Selecting this item allows you to display, on the same screen, the variation of Pressure, Temperature, Purity and Density with crankshaft angle for the box you select from the sub-menu list. This sub-menu will always contain the choices Cylinder and Crankcase. In addition, the choice Box Muffler is provided if the engine has a Box Muffler or the choice Int. Muffler for an engine with an Integrated Muffler and where the Box Model has been selected. The choice Induction is also provided for an engine with an Induction Box.

6.8 DUCT CHARACTERISTICS

This selection provides a sub-menu which lists the engine ducts. Clicking on a duct name provides, on the same screen, plots of the variation in duct Pressure, Temperature, Velocity and Purity with crankshaft angle, at either end of the selected duct. "INFLOW" refers to the end of the duct into which fluid would normally flow, whilst "OUTFLOW" refers to the end of the duct through which fluid would normally exit.

7 ... RUNNING THE ENGINE SIMULATOR

When Run the Simulator is selected from the Main MOTA Menu you are presented with a dialogue box which is different from the standard Windows example. The functionality of this box is the same as the Windows example, but it has one additional feature. By holding down the controlkey, you can select up to a maximum of twelve Engine Data Files in the chosen folder by clicking the mouse over them. When you click the OK button, all of the Engine Data Files you have selected will be processed consecutively by the engine simulator. Once the dialogue box disappears, the Engine Simulation screen should soon appear although on some lower speed computers this may take up to 20 seconds. You will notice that the title bar at the top of the screen is empty of all controls, so you are forced to interact with the simulator through the MOTA control buttons only. This is to prevent you from inadvertently shutting down the graphical environment whilst the simulation continues to run. If something should go wrong, you can close the simulation by clicking the right mouse button over the vbsim icon in the bottom tool bar and selecting "close".

During a simulation run, the name of the Engine Data File being processed is displayed beneath the various control buttons at the left hand side of the screen. Where more than one file has been selected for processing, the status of the entire selection is displayed under the headers Runs Completed, Current Run and Runs Queued.

Upon completion of a simulation run a message box is displayed at the top left hand side of the screen. This lists the names of the Engine Data Files which have been processed. At the same time the controls on the title bar of the Main MOTA Menu screen are re-activated.

During an engine simulation three control buttons are displayed which allow you to interact with the simulation. These are:

During the simulation, a number of key engine performance indicators is displayed numerically. These include the power, torque and the mean exhaust temperature (the average temperature at the centre of the exhaust duct over a complete revolution). As the number of revolutions rises, these performance indicators should approach limiting values. For a multi-speed engine simulation, once the simulation has been completed at two engine speeds, line plots of engine performance curves are displayed and updated dynamically throughout the remainder of the simulation. The units in which these line plots are displayed depend on the selection made on the Run Parameters edit screen discussed in section 4.6.1.

The simulator writes two output files to the same folder from which the data file in use by the simulator was obtained. Each of these has the same generic name as the data file, except with the extensions ".per" and ".gph". The ".per" file is the Engine Performance File. It contains text based information about the engineâ€s performance. The ".gph" file is the Graphics Output File which contains graphical representations of your engineâ€s performance. These files are discussed in detail in sections 5 and 6.

During a simulation comprising a large number of engine speeds, it is often impractical to watch the entire simulation to ensure that reasonable convergence has occurred at each speed. Also, such a visual check will not be possible with the faster processors which are becoming available. This is because several complete cycles will be performed by the simulator between each update of the screen display. The best way to check if convergence is acceptable is to view the Delivery and Exhaust Flow Ratios in the Engine Performance File output by MOTA. For each speed these should differ by less than 1%, although there may be 1 speed in 10 where this tolerance is exceeded. If more than 3 out of 10 speeds indicate a greater than 1% difference between the Delivery and Exhaust Flow Ratios, the value of the maximum number of revolutions should be increased and/or the value of the pipe step factor should be reduced on the engine data file Run Parameters screen; the simulation should then be re-run.

8 ... ENGINE DATA FILE CONSTRUCTION TOOLS

Selecting Tools from the Main MOTA Menu provides access to a number of utility routines:

  1. Piston Position to Crank Angle. Converts a piston distance from top dead centre (TDC) to a crank angle.
  2. Crank Angle to Piston Position. The inverse of (1).
  3. Chord Length to Arc Length and Arc Angle. Converts the "straight-line" distance between two points on the circle of a cylinder bore to the length of the arc joining the same two points. The arc angle and the chord length to bore ratio are also displayed.
  4. Arc Length to Chord Length and Arc Angle. Converts the arc length between two points on the circle of a cylinder bore to the length of the straight-line joining the same two points. The arc angle and the chord length to bore ratio are also displayed.
  5. Arc Angle to Chord Length and Arc Length. For the angle subtended at the centre of the circle of a cylinder bore by two points lying on the circle, the corresponding arc and chord lengths are displayed. The chord length to bore ratio is also displayed.
  6. Bridged Port Radial Attitude Angle. Calculates the effective radial attitude angle of a bridged port.
  7. Rectangular Duct Equivalent Diameter. Calculates the diameter of a circular duct with the same cross-section area as that of a given rectangular duct.
  8. Integrated Muffler Length. Calculates the length of an integrated muffler of defined muffler volume space.
  9. Integrated Muffler Volume. Calculates the muffler volume space of an integrated muffler. This value is required for the engine data file if the "box model" for the muffler is selected.
The suite of data file construction tools are provided in recognition of the fact that engine specifications are given in many different ways. You can enter as many combinations of input values to your selected tool as you wish. That is, for each of the various tools selections, the calculate button can be used as many times as you require. The screens provided by the various tool selections are largely self explanatory.

9 ... EXPANSION CHAMBER CONSTRUCTION UTILITIES

The selection of Expansion Chamber Construction from the Main MOTA Menu provides a sub-menu with the two choices:

Constructing the Development of a Cone

Printing the Development Pattern of a Cone

These are described separately below.

9.1 CONSTRUCTING THE DEVELOPMENT OF A CONE

This facility allows you to use expansion chamber data from an existing engine data file or from values you enter through the keyboard. It provides the dimensions of flat metal sheets which, when rolled and welded, allow the fabrication of the cones of a straight expansion chamber.

The selection of this option provides the sub-menu items:

The Development Diagrams Screen shows a drawing of a cone whose ends are square to its axis and a pattern of the sheet metal necessary to construct it. The symbols and nomenclature used in the two associated sub-menu items are also defined. This screen can be output to your printer as a reference.

The Enter Cone Values Manually option allows you to key in the two diameters and the length of a cone section and the thickness of the metal sheet you intend to use in its construction. The dimensions of the flat metal sheet necessary to construct the cone are then displayed and this information can be printed if required.

The Cone Values from an Engine Data File option allows you to select an existing MOTA data file which describes an engine with an expansion chamber. You are prompted for the thickness of the metal sheet you intend to use in the construction of this expansion chamber with any value in the range 0.4 to 2.0 mm accepted. The dimensions of the flat metal sheet suitable for the construction of each of the cones which make up the expansion chamber are then displayed and can be printed if required.

9.2 PRINTING THE DEVELOPMENT PATTERN OF A CONE

This facility is particularly useful where a cone with an angled end is to be constructed. In this case it is not easy to draw the development of the cone.

The selection of this option provides a sub-menu with the four choices:

Selecting Explanatory Diagrams provides a screen in which a selection of figures with text can be displayed.
Figure 1, A Cone Segment having Oblique Ends, shows the dimensions which must be provided before a pattern can be printed.

Figure 2, The Cut and Bend Angles, defines these two angles and shows that if a cone segment is cut into two pieces and then rejoined by welding after one of the pieces has been rotated through 180 degrees, the bend angle achieved is equal to twice the cut angle.

In Figure 3, Diameter at an Intermediate Length, the formula for evaluating the diameter of a cone at some intermediate point in its length is provided. It is noted that the evaluation of such a diameter can be performed on the Enter Cone Values Manually screen should this be required.

Selecting Enter Cone Values Manually provides a screen with prompts for the entry of the inside diameter and the cut angle at either end of a cone and the axial length of the cone. The thickness of the sheet metal to be used in the construction of the cone must also be entered. A scaled development of the cone can be displayed on the screen and a full size pattern can be printed. Such a pattern may extend over several A4 pages.
The diameter at an intermediate point in the length of a cone can also be evaluated on this screen. Typically, this tool is used to provide a value for entry as one of the end inside diameters as described above.

The Multi Piece Angled Cone selection can provide the patterns of cone pieces suitable for the construction of an angled section which is equivalent to a defined straight cone section. The dimensions of the straight section must be entered and then also the required total bend angle and the number of pieces to be used in the construction of the equivalent section. The pattern of each of the pieces or any subset of these pieces can be printed whilst a print screen content facility is provided in case a record of the displayed values is required.

Selecting Cone Values from an Engine Data File allows you to select a MOTA engine data file for an engine with an expansion chamber. You may then select any number of the expansion chamber sections for the output of a printed development pattern. You may also select any one section for which an equivalent bend section is to be constructed. In this case, after entering the total bend angle and the number of pieces to be used in the construction of the equivalent section, the pattern of each of these pieces is printed.

10 ... POWAPLOT

This allows you to construct and manipulate Power Files. These have the extension ".pow". In particular, Powaplot can be used to generate viewable graphs from dynamometer output. Such graphs can then be compared with other predicted and experimental power/torque curves using the overlay facility provided in Powaplot.

9.1 CONSTRUCT A POWER FILE FROM A GRAPHICS OUTPUT FILE

An engine simulation performed by MOTA which involves at least four engine speeds allows the display of power/torque curves when the associated Graphics Output File is accessed. This feature of Powaplot allows you to select a Graphics Output File and to nominate the number of cylinders in the complete engine. The power/torque curve information is then extracted from the ".gph" file, the power values multiplied by the number of cylinders and the values then written to a separate file having the same name as the source Graphics Output File but with the extension ".pow". The content of the new Power File is also displayed graphically in an environment identical to that of the power/torque option described in section 6.1, having precisely the same functionality as that option. The file can also be accessed as an overlay file both within the use of Powaplot and when displaying graphical information though a File Menu selection.

9.2 CONSTRUCT A POWER FILE MANUALLY

This enables you to construct a Power File by entering information on the screen which is displayed when you select this option.

The main data comprises an engine speed and a corresponding power or torque for as many speeds as you have selected in the first box on the screen. The minimum and maximum number of speeds are 4 and 40 respectively. The initial screen assumes the default options of imperial units, unequal speed intervals and power, but you may change these to suit the data you wish to input. If you select the equal speed interval option, you are presented with two boxes for you to enter the lowest speed and the speed interval; the boxes relating to your engine speed are then completed automatically so that you need only enter the corresponding power or torque values. If the unequal speed increments option is chosen, you are required to enter values of both the engine speed and the corresponding power or torque.

When you are satisfied with your input, click the Save as a File button and then select a drive and folder and provide a name for your file. The data you entered is then displayed graphically on the screen, together with the file construction screen. You may change the data on this screen and save to a new file or simply overwrite an existing file as often as you wish. Each time, the appropriate file is created and the corresponding graph is displayed. The display has the same functionality as the power/torque option described in section 6.1.

9.3 DISPLAY A POWER FILE

This enables you to select an existing Power File and display it in the same environment and with the same functionality as the power/torque option described in section 6.1. File selection is through the common dialogue box described previously.

11 ... DEMONSTRATION ENGINE DATA AND SIMULATION OUTPUT FILES

Nine sets of engine files are included with MOTA to enable users to become familiar with the capabilities of the program. These are:

For each of these engines, an Engine Data File, an Engine Performance File and a Graphics Output File are provided on the distribution media. The files are distinguished respectively by the extensions ".dat", ".per" and ".gph". The engine data files represent actual engines at various stages of their development .

Assorted additional Engine Data Files are included on the CD distribution disk, and loaded into the MOTA600 working directory at the time of installation of MOTA. They are identified by their name which generally corresponds to their commonly recognised model, and are supplied without any related MOTA output files.

CONVERSION FACTORS

LENGTH: 1 in = 25.4 mm
AREA: 1 sq.in = 6.4516 sq.cm
VOLUME: 1 cu.in = 16.387 cc
MASS: 1 lb = 0.45359 kg
POWER: 1 hp = 0.74570 kW
TORQUE: 1 ft lbf = 1.3558 Nm
VELOCITY: 1 ft/s = 0.30480 m/s
TEMPERATURE: degrees C = (degrees F - 32)/1.8
CALORIFIC VALUE: 1 BTU/lb = 0.55556 kcal/kg
FUEL CONSUMPTION: 1 lb/hph = 0.60828 kg/kwh
DENSITY: 1 lb/cu.ft = 16.0185 kg/m3
PRESSURE and YOUNG'S MODULUS: 1 psi = 6894.76 N/m2

EQUIVALENTS: 1 000 000 000 N = 1 GN
                              1 calorie = 4.1868 joules
                              1 N/m2 = 1 Pascal
                              1 atm = 101325 Pascals or 14.696 psi

CUSTOMER SUPPORT

All enquiries and requests for assistance may be directed:

by fax within Australia:  08 83409277
from Overseas:  (61) 8 83409277
by email:  iwt@iwt.com.au
Our main distributorâ€s website contains some more information concerning MOTA. Included are further technical explanations of some of the terms used in MOTA and links to related sites. Additional engine data files will be placed on the site as they become available to us, as will any other news we have concerning MOTA.

You can visit this website at:    http://www.iwt.com.au/mota.htm

DISCLAIMER

To the full extent permitted by State and Commonwealth Laws, any conditions or warranties imposed by legislation in relation to the use or supply of the MOTA software or manuals are hereby excluded. Insofar as liability under, or pursuant to, any legislation may not be excluded, the liability of each of the supplier and developer of the software is, at its entire discretion, limited to the replacement of the software or manuals. Except as provided above, each of the supplier and developer of the software shall not be liable to any person or organisation for any loss including economic loss, cost, damage or injury whatsoever and howsoever incurred and whether arising directly from the supply or use of the MOTA software or the manuals for such software.

TRADEMARKS

Windows, Windows 95, Windows 98, Windows 2000, Windows NT, Windows ME, Windows XP, Internet Explorer, Microsoft Word and WordPad are trademarks of Microsoft Corporation.
IBM is a registered trademark of International Business Machines Corporation.
Pentium is a registered trademark of Intel Corporation.
Hardlock is a registered trademark of Aladdin Systems.
MOTA is a registered trademark of IWT Racing.

DEDICATION

My contribution to MOTA is dedicated to my late wife Joyce.
Her encouragement and support were my extreme pleasure.
Malcolm.

ACKNOWLEDGEMENTS

This product would not have been possible without the contributions of the following:

The two men ultimately responsible for the majority of MOTAâ€s theoretical development and its user interfaces, Julian and Malcolm for their absolute dedication to the project. It simply would not have been possible without their skills.

Ian Williams for his leadership and drive in turning an esoteric, difficult to use computer program, into a user-friendly and useful commercial product.

Graeme for the name MOTA, and Richard and Lee for drawing the manualâ€s diagrams.

David Bowden, whose assistance enabled the completion of our first actual dyno, and gave the publisher the basic skills and an appreciation of the potential of the personal computer.

Michael and Tom Rogers for helping to produce everything from computer graphics and disc labels to actual working parts, and for performing on-track testing. (Our congratulations to Michael for becoming Australian National Superkart Champion in 2001)

Kym Rogers for all of his help, guidance, and practical knowledge in most every avenue which had to be investigated in order to create the package we now call MOTA.


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